linux-kernel - write_lock_irq(&tasklist

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Message-ID: <0879f797135033e05e8e9166a3c85628@codeaurora.org>
Date:   Tue, 22 May 2018 12:40:05 -0700
From:   Sodagudi Prasad <psodagud@...eaurora.org>
To:     keescook@...omium.org, luto@...capital.net, wad@...omium.org,
        akpm@...ux-foundation.org, riel@...hat.com, tglx@...utronix.de,
        mingo@...nel.org, peterz@...radead.org, ebiggers@...gle.com,
        fweisbec@...il.com, sherryy@...roid.com, vegard.nossum@...cle.com,
        cl@...ux.com, aarcange@...hat.com, alexander.levin@...izon.com,
        vegard.nossum@...cle.com, sherryy@...roid.com, fweisbec@...il.com,
        ebiggers@...gle.com, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: write_lock_irq(&tasklist_lock)

Hi All,

When following test is executed on 4.14.41 stable kernel, observed that 
one of the core is waiting for tasklist_lock for long time with IRQs 
disabled.
./stress-ng-64 --get 8 -t 3h --times --metrics-brief

Every time when device is crashed, I observed that one the task stuck at 
fork system call and waiting for tasklist_lock as writer with irq 
disabled.
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/kernel/fork.c?h=linux-4.14.y#n1843

Some other tasks are making getrlimit, prlimit system calls, so that 
these readers are continuously taking tasklist_list read lock.

Writer has disabled local IRQs for long time and waiting to readers to 
finish but readers are keeping tasklist_lock busy
for quite long time.

I think, −−get N option creates N thread and they make following system 
calls.
========================================================================
start N workers that call system calls that fetch data from the kernel, 
currently these are: getpid,
getppid, getcwd, getgid, getegid, getuid, getgroups, getpgrp, getpgid, 
getpriority, getresgid, getresuid,
getrlimit, prlimit, getrusage, getsid, gettid, getcpu, gettimeofday, 
uname, adjtimex, sysfs.
Some of these system calls are OS specific.
========================================================================

Have you observed this type of issues with tasklist_lock ?
Do we need write_lock_irq(&tasklist_lock) in below portion of code ? Can 
I use write_unlock instead of write_lock_irq in portion of code?
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/kernel/fork.c?h=linux-4.14.y#n1843

-Thanks, Prasad

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